This invention pertains to transducers for estimating the pressure applied to body tissue by an object such as a medical device or body part. The invention particularly pertains to a new and improved transducer having at least two plates separated from one another by an elastic element formed of rubber for measuring pressures acting normal to the plates by detecting the resultant change in separation of the plates. The invention is particularly useful in health care, for interposing at the surface between a tissue and an object without substantially displacing either the tissue or the object from the surface in order to estimate the pressure applied by the object at a predetermined location.
In medical tourniquet applications it is desirable to have a transducer that establishes the actual pressure applied by a device such as a cuff to the underlying tissue, in a continuous way, as the tourniquet cuff pressure varies during a medical procedure. Such a transducer must be of low profile and small relative to the size of the cuff so that it may be interposed between the cuff and the tissue without interfering with the function of the cuff or causing tissue damage.
Force and pressure are usually sensed using the xe2x80x98spring balance principlexe2x80x99, where, in a primary transducing step, an elastic element transforms the measurand into a deflection or deformation, which, in a secondary transducing step, is converted into an electrically useful signal. Common pressure transducers tend to depend on the deformation of a spring, cantilever, or diaphragm in the primary transducing step while electrical and optical techniques are used frequently in the secondary step.
The compliance of polymer materials and rubbers, in addition to their elasticity, makes them attractive candidate materials for the primary transducing step. Weighing mats by Miller et al. in U.S. Pat. No. 3,875,481 and load cell type transducers have been developed using elastomer elements where change in capacitance is used to infer the applied pressure. Improvements in relation to hysteresis and linearity of response have resulted from the use of voids and structured elastomer elements by Haberl et al. in U.S. Pat. No. 4,266,263 and by Seimiya et al. in U.S. Pat. No. 5,693,886. However, it is difficult to realize the optimum shapes without the use of complicated cutting and assembly procedures and individual device assembly. Low profile devices incorporating elastomer elements, designed for use as tactile transducers, have also been developed as disclosed for instance by Boie et al. in U.S. Pat. No. 4,526,043. However, the noise susceptibility of capacitor based transducers in general makes them less favorable for medical applications as either large areas or sophisticated support electronics are required in order to provide a satisfactory signal-to-noise ratio.
The noise problems associated with capacitive based devices can be circumvented to a large extent through the use of optical secondary transducing techniques. Pressure transducers having a flexible, pressure deformable reflector from which light radiation is reflected, are already well known as disclosed by Tenerz et al. in U.S. Pat. No. 5,195,375. Such devices have advantages for deployment in medical catheters for in situ physiological pressure measurements. However, they require sophisticated micromachining technology to form the flexible member, which is fragile and unsuitable for direct mechanical coupling.
In a biomedical pressure transducer disclosed by McEwen in U.S. Pat No. 4,869,265, a pressurisable chamber containing integral membrane switch type electrical contacts is interposed between the tissue and an apparatus such as a tourniquet cuff. The normally closed contacts are opened when the pressure within the chamber is equal to the pressure applied by the cuff. A fluid filled line is used to translate the pressure within the chamber to a detection device which is located remotely from the tourniquet site and thereby provide an estimate of the actual pressure applied by the cuff to the tissue. This approach does not provide a continuous dynamic estimate of the pressure applied and suffers from signal damping introduced by the fluid line.
An object of the present invention is to provide a transducer which will reliably and reproducibly measure the pressure applied by any one of a number of medical devices to a portion of a human body surface, tissue or organ. A related object is that the transducer should measure the pressure, applied by a specified device at a predetermined location relative to the device, in a direction normal to the plane of the device at the predetermined location. A further related object of the present invention is that it should measure the applied pressures in the range 0-500 mmHg (10 psi) and that it should measure such pressures near the location, averaged over an area no greater than 2 cm2.
A further object of the present invention is to provide a pressure transducer which is sufficiently small such that it does not introduce significant error by significantly altering the tissue/device interface during measurement A related object is that the transducer should not introduce significant error when used in measurement of pressures applied to curved surfaces in two dimensions having radii down to 2 cm.
Another object of the present invention is to provide a pressure transducer which is suitable for inclusion in a multi-transducer array. A related object is to provide a pressure transducer array which will conform to curved compliant tissue surfaces.
A further object of the present invention is to provide a transducer which does not require the use of electrical currents inside the body of the transducer. A related object is to provide a transducer which does not require the use of metal within the body of the transducer.
A further object of the present invention is to provide a transducer which permits fast, convenient calibration, or calibration checking, of the transducer in the application environment.
The present invention is provided to attain the above described objectives.
The present invention is embodied in a planar transducer for measuring biomedical pressures adapted for insertion between a tourniquet cuff and a human limb or body part to measure the pressure applied by the cuff to the limb or body part. The transducer comprises a pair of opposing plates, one a substrate plate and the other a pressure plate. It also includes a measurement channel optical fiber set with proximal end connected to the transducer and the distal end connected remotely to an electronics module. The electronics module contains the light source and photodetectors as well as support and proessing electronics. The measurement channel optical fiber set is comprised of two optical fibers, an emitter fiber, and a detector fiber for transmitting a measurement channel light beam to and from the transducer. A deformable polymer structure is located between the plates. When pressure is applied to the transducer, the deformation of the deformable polymer structure will result in a change in the degree of projection of a shutter-reflector into the measurement channel light beam. This will change the coupling of light between the emitter and detector optical fibers and thereby modulate the measurement channel light beam. A measurement photodetector located at the distal end of the measurement channel detector fiber produces an electrical signal which is representative of the intensity modulation of the measurement channel light beam. The reference channel provides an intensity reference signal. Both signals are amplified and passed to an analogue to digital (A/D) converter unit. The outputs of the A/D unit are in turn passed to a processor unit which provides an indication on the output device of the applied pressure.